FrogReaver
As long as i get to be the frog
Which means that first case cuts the raw incidents (as opposed to a mere slide along the percentage scale) of losing your concentration by 37.5%.
In that second case, it's cutting them by 60%. Pretty significant, in either case.
(Same principle as if you increase your crit chance from 5% to 10%, you are, in fact, doubling the number of crits you're getting.)
So? It's still 10% vs 4%. The point is that you get such a low chance of losing concentration anyways that you'll likely pass most of the saves anyways. The point is that there's going to be very few fights in your whole career where that +3 bonus to concentration saves actually prevented you from losing concentration.
By the time the bonus is a +4 or +5 and you are encountering enemies that can often enough do over 20 damage in a single hit then I can at least understand it's appeal. But at level 4? I don't get it.